Multiplying an object on a plain
The common solution for this is to use a particle system.  


Select your mountain  
Go into the particle system tab in the properties  
Set Physics to No, Render to "Object" and set your grass object as "Dupli Object" (if you got more than one grass object create a group and use this group instead)
Hit play for tiny second all your grass particles should now appear.  
Toy around with other setting or have a look into the blender documentation  
Go to the modifer tab and hit convert on the particle system to make all the particles into real objects